A 42-year-old man is intubated for status asthmaticus. He is deeply sedated and paralysed. Ventilator settings: VCV, TV 6 mL/kg IBW, RR 10/min, PEEP 0, FiO2 0.3, I:E 1:5. Peak pressure is 55 cmH2O, plateau pressure is 18 cmH2O. He is haemodynamically stable. Why is the RR intentionally low at 10/min?